Infrared Spectra: Primary Clues

Information on characteristic peaks associated with specific functional groups is tabulated below
and illustrated on the linked page for each group. [opens in same frame, use "back" to return here]
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Class of Compound
Functional Group
characteristic bands
 due to
~ peak location,
cm-1
peak description
& comments
Alkanes and
alkyl groups
C-H stretch

CH3 'umbrella'

CH2 scissor

to right of 3000

1375

1450

med-strong, med-broad

med-weak, sharp 

 usually med, sharp

Alkenes =C-H stretch

C=C stretch*

=C-H oop bend

to left of 3000

1600-1675*

650-1000

med, sharpish

med-weak, sharp

position can be correlated to pattern of substitution

Aromatics =C-H stretch
 

ring C=C stretch*
 

overtones
 

=C-H oop bend

to left of 3000
 

1450-1600
 

1667-2000
 

690-900

med, sharpish
 

sharp, weak
 

very weak, diagnostic for substitution pattern

strong, diagnostic for substitution pattern

Alkynes CC stretch*

C-H stretch

~2150*

~3300

weak, sharp

sharp?

Nitriles CN stretch 2300 medium, sharp
Alcohols O-H stretch

C-O stretch

3200-3600

1000-1200

 medium, broad

strong

Ethers C-O stretch 1000-1200 strong
Amines N-H stretch

N-H scissor

C-N stretch

3300-3500 

1560-1640 

1000-1350

broad; split for 1o, not for 2o

broad

med-weak, easy to miss

Aldehydes
C=O stretch*
 

 

CHO C-H stretch
 


1725*
 

 

2750 and 2850
 


strong, med-width

 

characteristic "sawteeth" on right of aliphatic C-H stretch region

Ketones C=O stretch* base value 1715* the C=O stretch is generally the strongest peak in any IR where it is present
Carboxylic Acids O-H stretch
 

C=O stretch*
 

C-O stretch

2500-3500


~1710


1200-1300

broad, may show "beard" of overshadowed C-H stretches

strong

medium, often hard to distinguish from other peaks in "fingerprint region"

       
Esters
C=O stretch*

 

C-O stretch

 


~1735

 

1000-1300 [2]

 

strong
 

fairly strong;
higher n due to sp2 C-O
lower n due to sp3 C-O

Amides C=O stretch*

C-N stretch
 

N-H stretch

N-H bend

1640-1670

1000-1350
 

3100-3500

1550-1640

strong

often lost in "traffic"

broad
 RCONH2 has split peak

 medium

Anhydrides C=O stretch*
 

C-O stretch

1740-1790 and
1800-1850

strong [as usual]

C-O bond has  more sp2 character than normal

1200-1300
Nitro N=O/N-O stretch 1300-1390 and
1500-1600
both strong
Halides C-F stretch

C-Cl stretch

C-Br stretch

C-I stretch

960-1350

500-800

to right of 667

to right of 667

strong, broad
       
* conjugation moves these peaks to the right [lower frequency]
increased ring strain when C=O is cyclic moves these peaks to the left [higher frequency]
See a graphic summary of where the peaks are
Test your skill in identifying some of the most characteristic IR peaks
Ronald W. Rinehart, 2002, 2003, 2004   "Actual results may vary."